Improved Analysis of Ultraviolet Absorption Spectra Under Nonequilibrium Shock Conditions

Abstract

A landmark set of measurements of high-temperature relaxation under strong shock heating conditions, conducted in 2013 by Ibraguimova et al. (“Investigation of Oxygen Dissociation and Vibrational Relaxation at Temperatures 4000–10800 K,” Journal of Chemical Physics, Vol. 139, No. 3, 2013, Paper 034317) using ultraviolet (UV) absorption converted to time-dependent vibrational temperature profiles, have been receiving considerable attention in the literature as a test of nonequilibrium thermochemical models. A number of attempts by computational fluid dynamics modelers to reproduce the published results were unsuccessful in predicting the peak vibrational temperature, the downstream slope, or both. Our recent analysis has shown that some of the assumptions used in spectra-to-temperature conversions, such as the two-temperature approximation, are violated in the early region of the shock, and a more detailed analysis and direct comparison of UV absorption, and not temperatures, is necessary. This work presents such an analysis where a gas number density, translational temperature, and distribution of rotational and vibrational level populations, obtained using a kinetic gas dynamic approach, are converted into UV in-band absorption for each temporal or spatial location in the shock using a detailed spectroscopic model. The computed UV absorption is then directly compared with the raw measured time-dependent UV absorption curves at several wavelengths. Such a comparison is shown to have more validity than the previous ones.